Three methods of determining a shaft's rotational speed are prevalent. They are: electromagnetic mini-generator; Reed switch transducer and Hall-Effect transducers. The Mini-gen-set devices are formed of a magnetic rotor surrounded by a plurality of coils each wound around a pole piece. As the rotor which is coupled to a shaft turns, a voltage is induced in each of the stator coils. These semi-sinusoidal voltages are added together and brought out as a signal which corresponds to the speed of shaft rotation.
Reed switch devices are formed of a reed switch placed in very close proximity to a rotating rotor which also contains one or more magnets. As the rotor which is coupled to a rotating shaft turns, the magnets on the rotor cause the reed switch contacts to close. When the magnet moves away from the switch, the switch contacts open. The switch contacts are usually connected to a suitable voltage source through a current limiting resistor. This results in an output signal that is essentially a square wave of constant amplitude and whose frequency is indicative of the speed of rotation of the shaft.
The third method of determining a shaft's speed of rotation is by means of a Hall-Effect transducer. This device also contains a rotor which is coupled to a shaft. Around the periphery of the rotor are individual magnets. Placed adjacent to the periphery of the rotor is a Hall-Effect sensor. A Hall-Effect device is a semiconductor material in which a current can be made to flow by placing it in a magnetic field. The stronger the field, the greater is the current flow. Placing the sensor in a field of opposite polarity will cause the current flow to cease. This current flow or lack of it is detected by appropriate circuits which are usually fabricated on the same substrate as the Hall-Effect sensing element and made available as a series of electrical pulses of constant amplitude having a frequency indicative of the rotational speed.
Each of the device types mentioned above has limitations. The mini-gen type of transducer is not completely compatible with digital electronic equipment due to its analog output signal. Furthermore, these devices do not operate satisfactorily at very low speeds. The Reed switch transducers suffer from contact bounce and have a limited mechanical life. The Hall-Effect device mentioned above is limited to the maximum number of magnetic poles that can be placed around the periphery of a rotor of a reasonable size.